Thin polished plates such as silicon wafers and the like are a very important part of modern technology. A wafer, for instance, may refer to a thin slice of semiconductor material used in the fabrication of integrated circuits and other devices. Other examples of thin polished plates may include magnetic disc substrates, gauge blocks and the like. While the technique described here refers mainly to wafers, it is to be understood that the technique also is applicable to other types of polished plates as well. The term wafer and the term thin polished plate may be used interchangeably in the present disclosure.
Semiconductor materials may be inspected for defects such as, e.g., surface imperfections, particles, irregularities the thickness of thin film coatings, and the like, which may hamper the performance of the semiconductor material. Some existing inspection systems direct a beam of radiation on the surface of the semiconductor material, then collect and analyze light reflected and/or scattered from the surface to quantify characteristics of the surface.
More specifically, existing systems generally utilize a beam scanner created by having a laser source with fixed output and adding an external scanning mechanism, such as an Acousto-Optic Device (AOD) scanner or a resonant scanner. Typical frequency converting laser sources use fixed spot design, where the source spot is focused on an oversized crystal, which is shifted to a fresh area when the exposed area has degraded to an undesirable level.
While the existing design works for lower power visible applications, high power ultraviolet (UV) and deep UV (DUV) applications create technical challenges related to the damage potential of scanning optics, which result in a life time and reliability issue of the light source itself. For instance, in case of an AOD, the limited optical efficiency of an AOD scanner (for example 50% for a dual AOD scanner at 266 nm) requires a very high power UV light source to make up for the efficiency loss. Currently, for example a few hundred mW laser power on the wafer is needed to fulfill the sensitivity requirement of scanning system, which means it requires more than 1 W laser source. Such a source has been considered technically extremely challenging until the very recent past and technical feasibility and life time is still in question. Future tool generations require even higher power levels, and the availability of such a laser is considered today a very high risk.
Therein lies a need for providing a laser source for inspection systems without the aforementioned shortcomings.